Modernity
Most of the cartridges you will come across, someone shows you, or you wind viewing in a gun store, are most likely modern cartridges. These cartridges work at operating pressures consistent with modern steel firearms. For the most part, they will use non-corrosive primers, and will be available in more than one loading. They will be reloadable, meaning that one can take the fired brass, clean it, and load it with new powder, seat a new primer and bullet, and create a new round (except for aluminum case ammunition, which is not reloadable, as well as rimfire ammunition). They will be loaded with modern gunpowder and will have precise tolerances. These cartridges are only a sliver of the mass of cartridges invented and sold since the dawn of metallic cartridges. A readily-available cartridge has stood the test of time either in terms of popularity of the cartridge (like the .45 ACP) or in terms of the sheer numbers of guns chambered for it in an earlier time (like the .32 SW Long and the .45 Long Colt until a few years ago).

Availability
Cartridges mentioned in gun magazines are widely available to the general population. Major gun manufacturers only produce weapons that have available ammunition. No one produces guns for obsolete ammunition; the demand just isn’t there. However, when buying a gun, give mind to the level of availability of the cartridge for it. A good deal on a .32 HR Magnum revolver may not be such a good deal when you find that local gun stores don’t stock .32 HR Magnum in your area. A .45 Long Colt single action revolver is a beautiful piece, but there are painfully few factory loadings for the .45 Colt.

Further Into the Topic
I have not even scratched the surface of this topic. I purposely stayed away from getting into the details of any particular cartridge. I discussed very few of the major exceptions to the rules. I didn’t go anywhere near the real meat of the topic. Of course, that was outside the scope of this work.

For further reading on this issue, treatments done by real professionals, and not an amatuer like me, try the following books. These are a few I pulled off my own bookshelf.

These books should be instantly familiar to anyone interested in firearms and ammunition. They are great reference material and enjoyable reads.

Diagrams
Note that lack of cannelure in the case body for the straight wall cartridge. The straight wall cartridge design is typical for revolver ammunition, the bottleneck for rifles. Both of these cartridge cases do not have bullets mounted in them.

The case head view is straight-on, looking at the bottom of the cartridge case. The primer sits in its pocket in the center. The stampings running around the case head indicate cartridge caliber. On military shells, the information is more cryptic.

The spitzer here has a boattail. Most high-velocity rifle bullets will. Older rifle bullets designs will not, however, many of these were created in the blackpowder era and thus were low-velocity shells. The wadcutter, roundnose, and semi-wadcutter bullets all have cannelures cut into their bodies. These cannelures will sit below the case mouth when they are loaded into a charged shell and will be invisible to the casual viewer.

The above diagram shows a slice of a completed handgun cartridge. The bullet’s cannelures are below the case mouth, normally hidden from view. The case is loaded with gun powder. This cartridge is straight walled and has a rimmed head.